A student dissolves ammonia gas in water to produce an alkaline solution. Explain why ammonia solution is described as a weak base, and describe how the pH of ammonia solution compares to that of sodium hydroxide solution of the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ammonia (NH₃) dissolves readily in water and acts as a base. Sodium hydroxide (NaOH) is also a base.
Model answer (5 marks)
Ammonia is a weak base because it only partially dissociates in water.
The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ lies to the left, so only a small fraction of NH₃ produces OH⁻.
Consequently the concentration of OH⁻ in an ammonia solution is low, giving a pH that is only slightly above 7.
Sodium hydroxide is a strong base; it fully dissociates to give a high concentration of OH⁻.
Therefore, for the same molar concentration, an ammonia solution has a lower pH than a sodium hydroxide solution.
The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ lies to the left, so only a small fraction of NH₃ produces OH⁻.
Consequently the concentration of OH⁻ in an ammonia solution is low, giving a pH that is only slightly above 7.
Sodium hydroxide is a strong base; it fully dissociates to give a high concentration of OH⁻.
Therefore, for the same molar concentration, an ammonia solution has a lower pH than a sodium hydroxide solution.
Examiner tips
- Use the term ‘weak base’ and explain partial dissociation. Mention the equilibrium lies to the left. State that OH⁻ concentration is lower. Compare to NaOH as a strong base. Conclude lower pH for ammonia.
Common mistakes
- Confusing ammonia with a strong base. Forgetting to mention the equilibrium direction. Saying the pH is higher instead of lower.
Mark scheme (5 marks)
- Ammonia is a weak base because it only partially dissociates (ionises) in water
- So the equilibrium in water lies to the left (reactants favoured)
- This means the concentration of hydroxide ions (OH⁻) in ammonia solution is low / lower than expected
- Sodium hydroxide is a strong base because it fully / completely dissociates in water
- Ammonia solution has a lower pH than sodium hydroxide solution of the same concentration (because it produces fewer OH⁻ ions)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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